The in vitro hydrolysis of leaf proteins. I.—The action of papain on protein extracted from the leaves of Zea mays

Maize leaf protein is not digested by papain at 37°, under conditions in which casein is hydrolysed. An increase in temperature has more effect on both the initial rate of reaction, and the final amount of hydrolysis, than an increase in enzyme concentration. Maximum digestion is observed at pH 6‐6 and 70° using KCN‐activated papain: 70% of the substrate N is hydrolysed to non‐protein N. De‐fatting of the protein by neutral solvent mixtures results in only a slight increase in digestion. Soluble N‐containing fractions precipitable by TCA occur at all pH values, but most at pH 7.5 and above. Digestion leads to almost complete solution of the leaf protein at pH 8.6 but the percentage of hydrolysis never exceeds that at pH 6.6. These soluble fractions resemble those found in similar digests of seed proteins. Though in vivo experiments show that protein made from mature wheat leaves has a higher nutritive value than protein from young leaves, there is no corresponding increase in the in vitro digestibility of protein extracted from maize leaves of increasing maturity.